In general there are several reasons why an item might not appear in a given search.
Too soon -- items can take 24 hours from the last revision for a listing to be indexed and appear in a search.
Sorting by "price plus shipping: lowest first" -- when a lowest first sort is performed, you will often see a message at the top of the results: "We removed some search results to show you the most relevant listings. View all results" -- if you want to see all the results, you need to either try another sort, or click on that "View all results" link.
Wrong Category -- if a site-wide search using a particular keyword returns many items, eBay will often direct that search automatically into one particular category. Sellers should take this into account when deciding which categories to list their items in. If possible, try searching the way a potential customer might, and see where the search ends up.
No Shipping -- if an item does not have a shipping method to a specified location, buyers in that location will not be able to see that item. This is often a problem for export sellers that do not ship to their own location.
Time Away -- if you put your listings on Time Away with certain settings, your items will be hidden from search. After ending a vacation it may take a day or two for listings to be re-indexed and appear again.
